As disclosed in, for example, Japanese Patent Application Laid-Open No. 59-228451 and Japanese Patent Application Laid-Open No. 2002-213803, a communication system of a so-called current loop method is conventionally known. With communication using the current loop method (hereinafter also “current loop communication”), a plurality of communication modules are provided in a loop communication line and each module changes the current flowing in the communication line to generate a transmission signal.
Each communication module has a phototransistor (phototransistor in a photocoupler) inserted in the communication line, and performs open/close control of the phototransistor to intermit a loop current, thereby generates a signal (current signal) utilizing the above change of the current.